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Timing of antibody testing in relation to onset of symptoms is key to detecting COVID-19

Tests to detect antibodies in peoples’ blood could show whether they currently have COVID-19 or have had it previously.

Scientists have analysed data from around the world to examine the accuracy of antibody tests for COVID-19 and have shown that the timing of testing is critical. The tests do not work accurately when administered at the wrong time.

In the most comprehensive review of its kind of COVID-19 testing, researchers have examined all available test accuracy evidence for antibody tests published up to the end of April. By combining data from many studies, they have shown that the tests are not accurate enough to confirm whether someone has the COVID-19 virus if used in the first two weeks after symptoms appear.

They found that tests are only likely to be useful in detecting previous SARS-CoV-2 infection if used at least 14 days after the onset of symptoms but say that studies show that the tests will miss 1 in 10 cases of COVID-19. However, they caution over-reliance on this figure, as the studies were small, poorly reported and done in select patient groups.

The researchers also fear accuracy will be lower when tests are used in the community, as the tests have mostly been evaluated in hospitalised patients – making it unclear whether they can detect lower antibody levels associated with milder and asymptomatic COVID-19 disease.

Accuracy of COVID-19 antibody tests

Led by experts at the University of Birmingham, a group of researchers drawn from universities around the globe have published their findings in the Cochrane Database of Systematic Reviews.

Jon Deeks, Professor of Biostatistics and head of the Test Evaluation Research Group at the University of Birmingham says: “Our systematic review has identified, appraised and synthesised all available data on the accuracy of antibody tests for detecting COVID-19 infection. We found clear evidence that the timing of testing in relation to the onset of symptoms is key. If antibody tests are used too soon after infection, there is a good chance of missing true cases. There is very limited evidence for the accuracy of these tests after five weeks.

“Most of the studies have been conducted in people hospitalised with COVID-19, probably with a high proportion of more severe cases and so we can’t be sure that these results will translate to people who had milder or even no symptoms, or to people who were infected more than five weeks previously.

“In spite of these limitations, antibody tests could be useful to confirm infection in people who still have symptoms after two weeks but have never had a swab test and to identify what percentage of people have previously had an infection so that the extent to which the infection has spread in the population can be estimated. At the level of the individual, although antibody tests can detect the previous infection, we don’t yet know if having antibodies provides protection from further infection, and if so for how long, so a positive antibody test does not mean that people can consider themselves immune.

The immune system of people who have COVID-19 responds to infection by developing cells that can attack the virus (antibodies) in their blood. Tests to detect antibodies in peoples’ blood could show whether they currently have COVID-19 or have had it previously.

Some tests use venous blood and specialist laboratory equipment, whilst others use finger-prick blood samples on disposable devices similar to pregnancy tests, which can be performed in laboratories, hospitals or at home.

“We have not yet been able to reliably identify whether any particular test or type of antibody test performs better than others, however, we will continue to update the review as more studies emerge in this fast-moving field,” Deeks adds.

“The design, execution and reporting of studies of the accuracy of COVID-19 tests require considerable improvement, and action needs to be taken to ensure that all results of test evaluations are available in the public domain and not withheld by test manufacturers to ensure we can provide the best estimates of the accuracy for these tests.”

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